This application claims the benefit of Korean Patent Application No. 2001-7321, filed on Feb. 14, 2001, which is hereby incorporated by reference as if fully set forth herein.
1. Field of the Invention
The present invention relates to a reflective liquid crystal display (LCD) device, and more particularly, to a reflective LCD device using a cholesteric liquid crystal color filter.
2. Discussion of the Related Art
Generally, thin film transistor-liquid crystal displays (TFT-LCDs) with high color quality and small depth dimension are mainly being used as LCDs.
Conventional LCD devices include a first substrate and a second substrate with a liquid crystal layer interposed therebetween. The first substrate and the second substrate are generally referred to as a color filter substrate and an array substrate, respectively.
The LCD devices use backlight sources disposed over the outer surface of the second substrate to provide light. However, only about 7% of the light that is emitted by the backlight passes through each cell of the LCD device. Since the backlight should emit light of a relatively high brightness, corresponding power consumption increases. Accordingly, a large capacity heavy battery is commonly used to supply sufficient power for the backlight. Moreover, use of the large capacity battery limits operating time.
Recently, to solve the above-mentioned problems, reflective LCD devices without the backlight are being researched and developed. Because power consumption of reflective LCD devices greatly decreases due to use of ambient light as a light source, operating time increases. Such reflective LCD devices are used for portable information apparatuses such as electric diaries and personal digital assistants (PDAs). In reflective LCD devices, a pixel area, which is covered with a transparent electrode in conventional transmissive LCD devices, is covered with a reflective plate or reflective electrode having opaque reflection characteristics. However, brightness of reflective LCD devices is very poor because the devices use only ambient light as a light source. The poor brightness results from operational characteristics of the reflective LCD devices in which ambient light passes through a color filter substrate, is reflected on a reflective electrode on a second substrate, passes through the color filter substrate again and then displays an image. Accordingly, brightness is decreased as a result of reduction of the transmittance when the ambient light passes through a color filter layer twice. Since overall thickness of the color filter layer is inversely proportional to transmittance and is directly proportional to color purity of the light, the problem of inadequate brightness of the reflective LCD devices can be remedied by forming a thin color filter layer with high transmittance and low color purity. However, there is a limit in fabricating the color filter layer below a threshold thickness due to characteristics of the resin used to form the color filter layer.
Accordingly, one possible solution to this problem is forming LCD devices using cholesteric liquid crystal (CLC) having selective reflection and transparency characteristics.
Generally, liquid crystal molecules have liquid crystal phases according to their structure and composition. The liquid crystal phases depend on the temperature and the concentration. Nematic liquid crystal material in which the liquid crystal molecules are aligned along one direction is mainly being researched and applied. Especially, the nematic liquid crystal material is nowadays being applied to the commercialized LCD. The CLC has an alignment state that the director of the nematic liquid crystal is distorted by mixing the nematic liquid crystal and the molecule whose molecular axis is distorted or whose reflected molecular phase has a different chiral characteristic from its original molecular phase.
The nematic liquid crystal phase has regularity that the liquid crystal molecules are aligned along one direction. In contrast with the nematic liquid crystal, the CLC has a structure of multi-layers, in each layer of which the liquid crystal has the regularity of the nematic liquid crystal. However, the liquid crystal alignment between the layers rotates, thereby the reflectance between layers being different. Therefore, the difference of the reflectance can display colors by reflection and interference of the light.
The rotation of the CLC molecule can be understood as a kind of helical structure. Two characteristics of the helical structure are a direction of rotation and a pitch that is repetition period of the helical structure.
The pitch can be understood as a distance through which the liquid crystal layer has the same alignment and is a variable to determine the color of the CLC. The central wavelength of the reflected light can be expressed the multiple of the pitch and the average refractive index (xcexc=navgxc2x7p). For example, if the pitch of the CLC whose average refractive index is 1.5 is 430 nm, the central wavelength of the reflected light is about 650 nm, thereby the reflected light being red. Similarly, green and blue can be displayed by making the CLC have an adequate pitch.
In the CLC structure, another important characteristic is the direction of the rotation, which is an important element to make polarization in the reflection characteristic of the CLC. The direction of circular polarization of the reflected light is determined according to direction of the helical structure of the CLC, i.e., left-handed or right-handed helical structure. For example, the CLC of the right-handed helical structure reflects the right-handed circular polarized light having the corresponding pitch.
The ambient light can be considered as a sum of the right-handed and left-handed circular polarized lights, and a specific circular polarized light can be divided by using the CLC. In the conventional LCD devices, the polarization (linear polarization) property is used. In the LCD devices using the CLC, since the practical use of the light is improved in contrast with the color filter using conventional pigments or dyes, the power consumption can be effectively decreased.
FIG. 1 is a schematic cross-sectional view of a conventional reflective LCD device using a CLC color filter.
A first substrate 10 and a second substrate 30 are facing and spaced apart from each other, and a liquid crystal layer 50 is interposed between the first and second substrates 10 and 30. On an inner surface of the first substrate 10, a first transparent electrode 12, which is a first electrode for applying a voltage to the liquid crystal layer 50, is formed. On the outer surface of the first substrate 10, a quarter wave plate (QWP) 14, that is, a xcex/4 retardation plate converting linearly polarized light into right- or left-handed circularly polarized light and conversely converting right- or left-handed circularly polarized light into linearly polarized light, is formed. Then, a polarizing plate 16 through which only light coinciding with the polarizing axis of the polarizing plate 16 can be transmitted is disposed on the QWP 14. Over an inner surface of the second substrate 30, a CLC color filter layer 32, which selectively reflects light of corresponding wavelength and transmits light of the other wavelength, is formed. The CLC color filter layer 32 composes a pixel xe2x80x9cPxe2x80x9d including three sub-pixels xe2x80x9cSxe2x80x9d of red (R), green (G) and blue (B). A light absorption layer 34 absorbing the transmitted light through the CLC color filter layer 32 is interposed between the CLC color filter layer 32 and the second substrate 30. A second transparent electrode 36, which is a second electrode for applying a voltage to the liquid crystal layer 50, is formed on the CLC color filter layer 32.
Accordingly, since the CLC color filter layer 32 not only displays colors but also reflects light, an additional reflecting plate is not necessary. As the reflecting plate much affects the reflective brightness in the reflective LCD device including the reflecting plate, so the CLC color filter layer affects the reflective brightness in the reflective LCD device using the CLC color filter.
FIG. 2 is a graph showing reflectance""s difference between conventional reflective LCD devices using a reflecting plate and a CLC color filter.
In FIG. 2, x-axis and y-axis represent wavelength and reflectance, respectively. T1, T2 and T3 are reflectance curves of the conventional reflective LCD device using the reflecting plate according to the colors R, G and B, respectively. The central wavelengths of T1, T2 and T3 are about 460 nm, 550 nm and 640 nm, respectively. The reflectance at each central wavelength of T1, T2 and T3 is about 0.95 and the difference between central wavelengths of T1, T2 and T3 is about 90 nm. C1, C2 and C3 are reflectance curves of the reflective LCD device using the CLC color filter without the reflecting plate according to the colors R, G and B, respectively. The central wavelengths of C1, C2 and C3 are nearly the same as those of T1, T2 and T3. However, the each bandwidth of C1, C2 and C3 is about 60 nm and the reflectance at each central wavelength of C1, C2 and C3 is about 0.5. That is to say, C1, C2 and C3 are narrower and lower than T1, T2 and T3. Therefore, the average reflectance of the reflective LCD device using the CLC color filter only has a range of about 0.1 to 0.15, while the reflectance of the conventional reflective LCD device using the reflecting plate is about 0.3.
On the other hand, for an absorptive color filter of the conventional reflective LCD device using the reflecting plate, since the thickness of the absorptive color filter is inversely proportional to the transmittance and proportional to the color purity, the transmittance and the color purity can be controlled by adjusting the thickness of the absorptive color filter layer. However, for the CLC color filter, the pitch of the CLC color filter is determined for the desired wavelength when the CLC color filter is designed and the bandwidth of reflectance curve for the CLC color filter is determined by the multiple of refractive index anisotropy of the CLC (xcex94n=ne-no) and the pitch (p), i.e. xcex94xcex=xcex94nxc2x7p. Therefore, after the CLC is completely aligned, each bandwidth for the colors R, G, and B is determined only by material property of the CLC. Additionally, since the reflectance of the CLC color filter varies according to the thickness of the CLC color filter, the thickness of the CLC color filter would be within a range of 2 to 5 micrometers for enough reflectance at each central wavelength. Consequently, for the CLC color filter different from the color filter of the conventional reflective LCD device, the reflective brightness cannot be controlled by the thickness of the CLC color filter.
The methods to increase the reflective brightness of the mono-layered CLC color filter can be divided into two types. The first method is to make the CLC incompletely aligned and the second method is to mix materials with different reaction properties and control the diffusion and reaction rate between the materials. By the methods, the bandwidth of the reflectance curve becomes wider so that the color purity can be decreased and the reflective brightness can be increased. For the first method, the reflectance at the central wavelength is decreased so that the increase in reflective brightness is not adequate. For the second method, each CLC color filter of colors R, G and B is very difficult to form in the color filter process because of the reaction mechanism of the CLC color filter.
Consequently, since the reflective LCD device using the mono-layered CLC color filter selectively reflects light of corresponding wavelength and transmits light of the other wavelengths, the color purity is high but the reflective brightness is low in contrast with the reflective LCD device using the reflecting plate.
Accordingly, the present invention is directed to a reflective liquid crystal display device that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide a reflective liquid crystal display device in which a cholesteric liquid crystal (CLC) color filter layer is multi-layered and has sub-color filters for red, green and blue colors to improve display quality by increasing a reflective brightness of a cholesteric liquid crystal color filter.
Another advantage of the present invention is to provide a reflective liquid crystal display device in which a cholesteric liquid crystal (CLC) color filter layer has sub-color filters for red, green and blue colors and each of the sub-color filters is divided into a plurality of regions to improve a display quality by increasing a reflective brightness of a cholesteric liquid crystal color filter.
Another advantage of the present invention is to provide a reflective liquid crystal display device in which a cholesteric liquid crystal (CLC) color filter layer is multi-layered and each layer of the cholesteric liquid crystal (CLC) color filter layer has sub-color filters for red, green and blue colors and each of the sub-color filter is divided into a plurality of regions to improve a display quality by increasing a reflective brightness of a cholesteric liquid crystal color filter.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. These advantages and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a reflective liquid crystal display device comprises first and second substrates facing and spaced apart from each other; a first transparent electrode beneath the first substrate; a light absorption layer on the second substrate; a cholesteric liquid crystal color filter layer on the light absorption layer, the cholesteric liquid crystal color filter layer being multi-layered and each layer of the cholesteric liquid crystal (CLC) color filter including sub-color filters for red, green and blue colors arranged in an alternating order; a second transparent electrode on the cholesteric liquid crystal color filter layer; and a liquid crystal layer interposed between the first and second transparent electrodes. The reflective liquid crystal display device of the present invention further includes a plurality of thin film transistors, which correspond respectively to each of the sub-color filters and are connected to the corresponding first transparent electrode, beneath the first substrate and a passivation layer beneath the thin film transistor and the first substrate. However, the plurality of thin film transistors, which correspond respectively to each of the sub-color filters, may alternatively be formed on the second substrate and connected to the corresponding second transparent electrode. The reflective liquid crystal display device of the present invention further includes a quarter wave plate on the first substrate and a polarizing plate on the quarter wave plate. The multi-layered cholesteric liquid crystal (CLC) color filter layer consists of at least two layers.
In another aspect, a reflective liquid crystal display device comprises first and second substrates facing and spaced apart from each other; a first transparent electrode beneath the first substrate; a light absorption layer on the second substrate; a cholesteric liquid crystal color filter layer on the light absorption layer, the cholesteric liquid crystal color filter layer including sub-color filters for red, green and blue colors arranged in an alternating order, each of the sub-color filters being divided into a plurality of regions; a second transparent electrode on the cholesteric liquid crystal color filter layer; and a liquid crystal layer interposed between the first and second transparent electrodes. The reflective liquid crystal display device of the present invention further includes a plurality of thin film transistors, which correspond respectively to each of the sub-color filters and are connected to the corresponding first transparent electrode, beneath the first substrate and a passivation layer beneath the thin film transistor and the first substrate. However, the plurality of thin film transistors, which correspond respectively to each of the sub-color filters, may alternatively be formed on the second substrate and connected to the corresponding second transparent electrode. The reflective liquid crystal display device of the present invention further includes a quarter wave plate on the first substrate and a polarizing plate on the quarter wave plate. Each regions of the sub-color filters having a different central wavelength of reflection. The central wavelength of reflection for red color is between 620 and 650 nm, for green color between 530 and 570 and for blue color between 440 and 480.
In another aspect, a reflective liquid crystal display device comprises first and second substrates facing and spaced apart from each other; a first transparent electrode beneath the first substrate; a light absorption layer on the second substrate; a cholesteric liquid crystal color filter layer on the light absorption layer, the cholesteric liquid crystal color filter layer being multi-layered and each layer of the cholesteric liquid crystal (CLC) color filter layer including sub-color filters for red, green and blue colors arranged in an alternating order, each of the sub-color filters being divided into a plurality of regions; a second transparent electrode on the cholesteric liquid crystal color filter layer; and a liquid crystal layer interposed between the first and second transparent electrodes. The reflective liquid crystal display device of the present invention further includes a plurality of thin film transistors, which correspond respectively to each of the sub-color filters and are connected to the corresponding first transparent electrode, beneath the first substrate and a passivation layer beneath the thin film transistor and the first substrate. However, the plurality of thin film transistors, which correspond respectively to each of the sub-color filters, may alternatively be formed on the second substrate and connected to the corresponding second transparent electrode. The reflective liquid crystal display device of the present invention further includes a quarter wave plate on the first substrate and a polarizing plate on the quarter wave plate. The multi-layered cholesteric liquid crystal (CLC) color filter layer consists of at least two layers. Each regions of the sub-color filters having a different central wavelength of reflection. The central wavelength of reflection for red color is between 620 and 650 nm, for green color between 530 and 570 and for blue color between 440 and 480.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.